There are track loaders described in Japanese Unexamined Patent Publications JP-A 2010-270527 and JP-A 2010-270528 as a self-propelled working machine.
The track loader has a HST (Hydro Static Transmission) including: a HST pump composed of a tilted-plate type variable capacity pump driven by an engine; and a HST motor for driving a travelling device driven by a discharge oil from the HST pump, the HST motor being connected to the HST pump to be in a closed circuit with use of a pair of gear-changing oil passages.
In addition, the track loader has a main pump and a pilot pump each driven by the engine, and has: a working device driven by a discharge oil of the main pump; and a travel operation device for controlling a tilted plate of the HST pump due to a discharge oil of the pilot pump.
The working device has: an arm operated to be swung upward and downward; and a bucket attached to a leading end side of the arm.
As for the track loader, in a case where a load is applied to the HST motor such as in a case where the bucket is thrust into, for example, earth and sand by moving the track loader forward, the load applied to the HST motor is transmitted via the HST pump to the engine, and accordingly an engine speed is dropped.
On this occasion, when the bucket is operated to shovel at the same moment and thus a high load is applied to the bucket, the load acted from the bucket on the main pump may be transmitted to the engine to stall the engine, and accordingly in the conventional track loader, in order to prevent the engine from stalling, a bleed circuit for draining a part of the pilot oil supplied from the pilot pump to the travel operation device via a throttling is provided.
Due to the provision of the bleed circuit, when the engine speed is dropped, the number of rotations of the pilot pump is decreased to decrease a discharge amount of the pilot pump, and a ratio of an oil leak from the bleed circuit with respect to the discharge amount of the pilot pump becomes large. And accordingly, a primary side pressure of the travel operation device (that is referred to as a travel primary side pressure) decreases, a pilot pressure outputted from the travel operation device decreases in accordance with the dropping of the engine speed, and thereby a tilted-plate angle of the HST pump is automatically adjusted so as to reduce the rotation number of the pump (so that the tilted plate can return to a neutral side) to decrease the load of the engine, thus preventing the engine from stalling.